Reverse osmosis (RO) desalination uses membrane technology to transform seawater and brackish water into fresh water for drinking, irrigation and industrial applications. Reverse osmosis desalination processes require substantially less energy than thermal desalination processes. As a result, the majority of recent commercial projects use more cost-effective reverse osmosis membranes to produce fresh water from seawater or brackish water. Over the years, advances in membrane technology and energy recovery devices have made reverse osmosis more affordable and efficient. Despite its capacity to efficiently remove ionic species at as high as 99.8% salt rejection, there remains a need for reverse osmosis membranes that possess improved flux characteristics while maintaining useful rejection characteristics.
Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute. The membranes used for reverse osmosis have a dense barrier layer where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). Examples of reverse osmosis processes are the purification of brackish water and seawater, where often less than 1% of the impurity species in the seawater or brackish water are found in the permeate. The reverse osmosis process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
Nanofiltration, in concept and operation, is much the same as reverse osmosis. The key difference is the degree of removal of monovalent ions such as chlorides. Reverse osmosis removes about 99% of the monovalent ions. Nanofiltration membranes removal of monovalent ions varies between 50% to 90% depending on the material and manufacture of the membrane. Nanofiltration membranes and systems are used for water softening, food and pharmaceutical applications. An example of a nanofiltration process is the desalting of a sugar solution, where 80% of the salt passes through the membrane with the water and 95% of the sugar is retained by the membrane.
It is well known that for a given polymer, there is a flux-rejection trade-off curve that defines the upper bound of the flux-rejection relationship. One can obtain high membrane flux with trade-off in terms of salt rejection. On the other hand, one can obtain high membrane salt rejection with trade-off in terms of membrane water permeability. It is highly desirable to obtain membrane materials with performance above the trade-off curve, i.e., achieving both high flux and high salt rejection.
Nanotubes such as carbon and boron nanotubes are fundamentally new nanoporous materials that have great potential for membrane applications. The current methods of synthesis of CNT membranes (Hinds et al Science, 2004; Holt et. al. Science, 2006; Formasiero et. al., PNAS, 2008) involve multiple steps and are limited to making membrane samples of extremely small area. They are not scalable to large surface areas necessary for the fabrication of commercial membranes for practical applications. Membranes containing carbon nanotubes have been disclosed for use in purifying water. For example, WO 2006/060721, assigned to National University of Singapore, describes thin film composite (TFC) membranes containing multi-walled carbon nanotubes (MWNT) in an active layer prepared by interfacial polymerization. The MWNTs are characterized as having an outside diameter of 30-50 nm. However, further improvements in the performance of TFC membranes for reverse osmosis applications are desirable.